ci: add scripts to install Emscripten 4.0.9

.gitlab/.gitignore

@@ -2,6 +2,7 @@
 /5.15.1-0-202009071110*
 /bcc*
 /cmake*
+/emsdk
 /iar
 /intel
 /ispc*

.gitlab/ci/emsdk-env.sh

@@ -0,0 +1,3 @@
+.gitlab/ci/emsdk.sh
+. .gitlab/emsdk/emsdk_env.sh
+em++ --version

.gitlab/ci/emsdk.sh

@@ -0,0 +1,25 @@
+#!/bin/sh
+
+set -e
+
+case "$(uname -s)-$(uname -m)" in
+    Linux-x86_64)
+        ;;
+    *)
+        echo "Unrecognized platform $(uname -s)-$(uname -m)"
+        exit 1
+        ;;
+esac
+
+cd .gitlab
+
+version=4.0.9
+dirname="emsdk-$version"
+filename="$dirname.tar.gz"
+curl -OJL "https://github.com/emscripten-core/emsdk/archive/refs/tags/$version.tar.gz"
+tar xzf "$filename"
+mv "$dirname" emsdk
+emsdk/emsdk install "$version"
+emsdk/emsdk activate "$version"
+
+rm -f "$filename"